The present invention relates to a lining structure composition for the inner surface of a tundish characterized by forming an alumina-silica base intermediate lining layer on which a layer of a magnesian gunning material is coated.
In the past, the inner surface of a tundish for steel production was usually lined with refractory bricks and/or casting materials made of high silicious, chamotte, alumina, zircon group materials on which a surface coating layer was formed with a trowelling material or a board consisting of a magnesian substance for the purpose of avoiding foreign materials in the steel products.
Application by gunning has recently been developed for the purpose of saving manual labour wherein the same type of magnesian materials are employed as for trowelling or application with boards.
However, the corrosion resistance of a surface coating layer applied by gunning sometimes rapidly deteriorates, the layer partially vanishing, and even the refractory lining under the surface covering layer may also be damaged after operation of a tundish for a long period of time. On the other hand even if the corrosion resistance of the surface coating layer does not deteriorate after long operation of a tundish, the surface coating layer and the refractory lining material beneath it may strongly adhere to each other at the time of taking out residual steel and skull after the operation of a tundish. This adhesion not only makes the removal of residual steel and skull difficult but also makes it necessary to remove the adhered layers, which was unnecessary with a tundish lined with a trowelling material or boards.
When a magnesian board or trowelling material is employed for the surface coating application of a tundish, the above-mentioned problems are not caused. Such inconveniences arise only when a surface coating is formed by gunning application. When a surface coating is applied by gunning, the coating material deeply penetrates into the minute pores and uneven portions of the surface of a refractory lining material, establishing a high bonding strength and strongly adhering to the refractory material.
While this advantageously reduces peeling off of the coating from the lining during the operation of a tundish the adhesion of coated material to the refractory lining is so solid at the operating temperature of the tundish (about 1550.degree. C.), that both materials react easily.
If a refractory surface coating material the same in quality as the refractory lining material is employed, the above-mentioned defect resulting from the reaction can be avoided. However, when an alumina-silica-base or zircon-base material is employed for a surface coating layer, such materials cause contamination in molten steel as Si and/or non-metal inclusions. Magnesia-base material is usually employed for the surface coating layer with the thickness range of 5 to 40 mm to avoid the above problem.
The degree of reaction between a surface coating gunning material and a refractory lining material was inspected by analyzing a piece of surface coating layer after operation of a tundish. As a result, it was found that the SiO.sub.2 component increased in a surface coating layer when a high silicious or a chamotte material was used as a refractory lining material. It is thought that when a refractory lining material rich in SiO.sub.2 component and a surface coating gunning material comprising magnesian material are exposed at a high temperature during the operation of a tundish, they react, resulting in the migration of SiO.sub.2 contained in the refractory lining material into the surface coating layer, and the refractoriness of the surface coating layer is reduced while its corrosion resistance simultaneously deteriorates. On the other hand, when a refractory lining material consists of a medium alumina, a high alumina or zircon material, little migration of components contained in the refractory lining material into the surface coating layer occurs, and it was found that a thin vitreous layer and a recrystallized product are formed on the interface between the surface coating layer and the refractory lining material by the reaction between both materials, and that the surface coating layer and the refractory lining material adhere strongly.